Copiers and printers are undergoing device downsizing and enhancements in energy efficiency, and magnetic monocomponent development systems that use a favorable magnetic toner are preferably used in this context.
In a magnetic monocomponent development system, development is carried out by transporting a magnetic toner into the development zone using a toner-carrying member (referred to below as a developing sleeve) that incorporates in its interior means of generating a magnetic field, e.g., a magnet roll. In addition, charging of the magnetic toner is carried out, mainly in the zone where the magnetic toner is subjected to control by a toner control member (also referred to herebelow as a developing blade), by tribocharging due to rubbing between the magnetic toner and a tribocharge-providing member, e.g., a developing sleeve. Reducing the size of the developing sleeve is an important technology in particular from the standpoint of reducing the size of the device.
Given these circumstances, fixation tailing is an image defect of concern for the future. This problem is caused when the toner is blown off due to the generation of a current of water vapor from the recording medium, e.g., paper, when heat is applied in the fixing section. This problem is prone to appear, for example, with images that tend to have a high toner laid-on level, e.g., horizontal line images. This problem also readily occurs in cases of a large current of water vapor produced from the recording medium. As a consequence, this problem tends to get even worse in high-temperature, high-humidity environments.
There has been much research on this image defect known as fixation tailing, with a focus on engineering the low-temperature fixability of magnetic toner and on engineering the charging performance of magnetic toner.
In Patent Document 1, the attempt is made, by improving the low-temperature fixability of the magnetic toner, to inhibit spots around line images and to improve the fixation tailing with magnetic toners.
In Patent Document 2, the attempt is made to raise the electrostatic adsorptive force to the recording medium and improve fixation tailing by improving the charging performance of the parent material of the magnetic toner.
In Patent Document 3, the attempt is made to improve the fixation tailing by controlling the charging performance of external additives and controlling the releasability by the magnetic toner from the photosensitive member.
A certain effect on the fixation tailing is in fact seen due to engineering of the low-temperature fixability of magnetic toner and engineering of the charging performance of magnetic toner. However, it is thought that there is room for additional improvement in order to satisfy the needs of recent years.
Several factors as described in the following may be offered as possible causes here.
(1) When the diameter of the developing sleeve is reduced in order to reduce the size of the printer body, it becomes difficult to secure and maintain the charging performance of the magnetic toner, which as a consequence impairs the ability to inhibit the appearance of fixation tailing. The reason for this is as follows: with a reduced-diameter developing sleeve, the zone (referred to below as the control nip) in which the toner control member in contact with the developing sleeve can control the toner laid-on level is narrowed and a tendency appears for the amount of charge on the magnetic toner to decline.
(2) Lowering the fixation temperature in pursuit of greater energy savings for a printer works against inhibiting the appearance of fixation tailing. In particular, fixation tailing readily undergoes additional increases when the recording medium is a so-called heavy paper that impedes the propagation of heat from the fixing unit to the magnetic toner.
Given this background, there was still room for investigations in order to improve the fixation tailing.